1. Field of the Invention
The present invention relates to a photograhic silver halide emulsion which is spectrally sensitized with a spectral sensitizing dye, more particularly, to a silver halide photographic emulsion which is spectrally sensitized in the red wavelength region.
2. Description of the Prior Art
As one technique of making light-sensitive photographic materials, it is well known that the light-sensitive wavelength region is further widened to a longer one by incorporating certain cyanine dyes into a silver halide photographic emulsion. This is called spectral sensitization.
However, spectral sensitizing effects are influenced by photographic additives such as a stabilizing agent, an anti-fogging agent, a coating aid, a precipitating agent, color image forming couplers, etc., which are often added to the emulsion in combination therewith. Among these, particularly, color image forming couplers have a large influence upon spectral sensitization, since the couplers are used in a large amount.
It is required that spectral sensitizing dyes used for spectral sensitization of photographic silver halide emulsions have no undesirable interaction with other photographic additives and have stable photographic properties during storage of light-sensitive materials.
Further, such spectral sensitizing dyes are required to cause no residual color in processed light-sensitive materials. It is particularly required that there be no residual color after rapid processing, usually carried out for from several ten seconds to several hundred seconds.
On the other hand, various measures have been suggested to obtain superior color reproduction in color light-sensitive materials. One technique which has recently been carried out to obtain excellent color reproduction by using only one kind of light-sensitive material for various light sources (for example, sun light, tungsten light, an electronic flash, a fluorescent light or a combination thereof) is disclosed in Japanese Patent Publication No. 6207/74, where the maximum spectral wavelength of each of various color light-sensitive layers is set to minimize changes in color hue resulting from the use of different light sources; it is also desirable that the maximum spectral wavelength of the red-sensitive emulsion layer be at 610 nm (.+-.5 nm). However, where the maximum spectral wavelength of the red-sensitive layer is shortened as above, sensitivity, in general, is lowered. For instance, where a light-sensitive material having maximum sensitization at 610 nm is employed, it is difficult to obtain the same red sensitivity as produced by a light-sensitive material having maximum sensitization at 630 nm or 640 nm.
In order to obtain the same sensitivity, it is required that the grain size be enlarged to raise the sensitivity of the original emulsion. Enlarging the grain size brings about disadvantages such as a decrease of image sharpness and so on. Reproduction of flesh tones, which is important in color photography, has a close relationship to the maximum spectral wavelength of the red-sensitive layer. It is undesirable to shorten the wavelength and it is desirable that the maximum spectral wavelength be longer than 625 nm.